C ***************************************************************

      real function calctht0 (bszlyd, thetes, thetar,
     i                      theta, thetaw,awct, ephc, eahc)

c     + + + PURPOSE + + +

c     calctht0 - calculate relative amount of surface water

c     + + + ARGUMENT DECLARATIONS + + +

      real bszlyd(*)
      real thetes(*)
      real thetar(*)
      real thetaw(*)
      real theta(0:*)
      real awct
      real*8 eahc, ephc

c     + + + ARGUMENT DEFINITIONS + + +

c     bszlyd - depth of layers
c     theta  - water content (m^3/m^3)
c     thetes - reduced saturation content (m^3/m^3)
c     thetar - reduced wilting point content (m^3/m^3)
c     thetaw - wilting point (m^3/m^3)
c     awct   - available water content (m^3/m^3)
c     ephc   - potential soil surface evaporation (mm)
c     eahc   - actual soil surface evaporation (mm)

c     + + + LOCAL VARIABLES

c     thetax - extrapolated surface soil water content
c     eratio - Ratio of actual to potential bare soil evaporation
c     thetae - Equivalent surface soil water content (m^3/m^3)
c     theter - Surface soil water content based on relationship
c              between evaporation ratio & equivalent soil water
c              content (m^3/m^3)

      real thetax
      real eratio
      real thetae
      real theter

c     + + + CALLED FUNCTIONS + + +

      real extra

c     + + + END SPECIFICATIONS + + +

      thetax = extra(bszlyd, thetes(1), thetar(1), theta)		!h-64,65,66

      if ( theta(1).gt.(thetaw(1)+awct*.70) ) then
         calctht0 = max(thetax, theta(0))
      else
         if ( ephc.le.0.0 ) then
            calctht0 = max(thetax,theta(0))
         else
            eratio = eahc/ephc									!text after h-85
c     This function estimates soil wetness at the soil-atmosphere
c     interface based on a sigmoid curve that describes the relationship
c     between evaporation ratio and surface soil wetness expressed as
c     equivalent water content.

            thetae = 0.24308+1.37918/
     *               (1.0+EXP(-(eratio-0.44882)/0.081))					!h-85
            theter = thetae*thetaw(1)
            calctht0 = min(theter,thetax,theta(0))
c ***            write(*,*) 'calctht0:thetae,eratio,theta(0)',thetae,eratio,
c ***     *                 theta(0)
         endif
      endif

C ***      if (theta(1) .gt. ( thetaw(1) + awct*.70 )) then
C ***         if ( ephc .eq. 0.0 ) then
C ***            theta(0)= max(thetax , theta(0))
C ***            go to 290
C ***         else
C ***            theta(0)= thetax
C ***            go to 290
C ***         end if
C ***      end if
C ***
C ***      if ( ephc .gt. 0.0 ) then
C ***         eratio = eahc/ephc									!text after h-85
C ***
C ***c     This function estimates soil wetness at the soil-atmosphere
C ***c     interface based on a sigmoid curve that describes the relationship
C ***c     between evaporation ratio and surface soil wetness expressed as
C ***c     equivalent water content.
C ***
C ***         thetae = 0.24308+1.37918/
C ***     *            (1.0+EXP(-(eratio-0.44882)/0.081))					!h-85
C ***         theter = thetae*thetaw(1)
C ***         theta(0) = min(theter,thetax,theta(0))
C ***      else
C ***         theta(0) = max(thetax/2.0,theta(0))
C ***      end if
C ***
      return
      end